While an air conditioner is performing a heating operation, frost formation can occur in an outdoor heat exchanger functioning as an evaporator. Frost formation in the outdoor heat exchanger raises a fear that such frost can obstruct ventilation in the outdoor heat exchanger to thereby lower the heat exchange efficiency of the outdoor heat exchanger. Thus, there is proposed an air conditioner which, when frost is formed in the outdoor heat exchanger, interrupts a heating operation and, by switching a refrigerant circuit to allow the outdoor heat exchanger to function as a condenser, applies a high-temperature refrigerant discharged from a compressor to the outdoor heat exchanger to thereby perform a defrosting operation to melt the frost attached to the outdoor heat exchanger (see, for example, JPA publication No. 2009-228928).
In the above air conditioner, frost formation in the outdoor heat exchanger during the heating operation is checked using values detected by various temperature sensors or the duration of the heating operation. For example, when the temperature of a refrigerant flowing out from the outdoor heat exchanger during a heating operation, namely, the heat exchanger outlet temperature is lower than −14° C., or when a temperature difference between the outdoor air temperature and heat exchanger outlet temperature is larger than 5° C., or when the duration of the heating operation exceeds three hours, etc., the amount of frost formed in the outdoor heat exchanger is determined to be a level interfering with the heating capacity, thereby executing a defrosting operation.
Meanwhile, when the outdoor air temperature during the heating operation is low and the humidity of the outdoor air is high, frost formation can progress suddenly in the outdoor heat exchanger (a large amount of frost can attach to the outdoor heat exchanger for a short time). Such sudden frost formation progress cannot be detected by the above-mentioned frost formation checking method. Then, it is expected that frost formation sudden progress in the outdoor heat exchanger is checked by detecting the sudden temperature drop of the outdoor heat exchanger. For example, supposing the heat exchanger outlet temperature is checked periodically (for example, every five minutes). In the case that the checked temperature is lower than −6° C., when the drop rate of the heat exchanger outlet temperature obtained by subtracting the current heat exchanger outlet temperature from the previously (five minutes earlier) detected exchanger outlet temperature is larger than 2° C./5 min., it is determined that frost formation has progressed suddenly in the outdoor heat exchanger, thereby executing the defrosting operation.
In an air conditioner with a plurality of indoor units connected to an outdoor unit, during the heating operation, the number of indoor units in operation can increase (indoor unit(s) not operated so far can start heating operation(s)). When the number of indoor units in operation increases, the number of rotations of a compressor is increased according to the increased number of indoor units. The increased value of the then rotation number, even when only one indoor unit is increased, is larger than a value when a normal required capacity is changed (for example, when the set temperature in the indoor unit is increased by 1° C.). And, when the rotation number increased value of the compressor is large, the suction pressure of the compressor drops greatly and the heat exchanger exit temperature also greatly drops accordingly. Therefore, during the heating operation, when the number of indoor units in operation increases while checking from the above-mentioned drop rate of the heat exchanger outlet temperature whether frost formation has suddenly progressed in the outdoor heat exchanger or not, the drop rate of the heat exchanger outlet temperature increases although frost formation has not actually progressed suddenly in the outdoor heat exchanger, thereby raising a fear of determining in error that frost formation has progressed suddenly.